• 4.2 baseline — 0.6 × season 4.4 (18 gm) + 0.4 × last 10 3.9
• +0.0 opponent leak — they concede 94/game vs a 93.4 league average
• −0.2 at this ground — averages 4 there (48 gm)
• vs them: averages 3.9 over 9 games — evidence, never summed

Derived, not measured — plain arithmetic over his real games, shown in full. Adjustments can overlap (a game at this ground can also be a wet one); they add simply, and this note is the admission.